The present invention relates to fully dense filamentary reinforced monotapes and more particularly to a method of producing long-length or large-sized monotapes.
High modulous, high strength metal matrix composites have become increasingly important in the fabrication of parts such as blades, disks and spacer rings, for steam or gas turbine engines, propellers, and a wide variety of other special applications in both the aerospace and other industries. In general, these monotapes exhibit ansiotropic strength characteristics, which generally include high stiffness, high strength, and low density. The strength and the stiffness are directly related to the high tensile strength of the reinforcing filaments and are therefore substantially greater in the direction parallel to the plane in which the majority of the filaments are aligned.
The specific details of monotapes and their fabrication processes may vary from manufacturer to manufacturer, however, in general the monotape consists of a sandwich of one or more layers of collimated fibers or reinforcing filaments positioned in a metal alloy matrix material. The reinforcing filaments are generally in the form of a "mat" prealigned in a single plane, carefully spaced parallel to each other, with very widely spaced metal ribbons, a binder, or perpendicular cross wires, typically as illustrated in FIG. 1 of U.S. Pat. No. 4,260,441. The sandwich of filaments and matrix material is subjected to a high temperature, a high pressure compacting procedure, usually either hot isostatic pressing or press diffusion bonding, to produce the compacted monotape.
The overall strength characteristics of the monotape will, of course, be primarily a function of the high strength reinforcing filaments employed. These typically include materials such as filaments of boron, boron coated with silicon carbide or boron carbide, silicon carbide, ceramics such as alumina, and refractory metals such as tungsten and molybdenum. Typical matrix alloys are aluminum, magnesium, titanium, iron-base superalloys, nickel-base superalloys, and intermetallic compounds such as titanium aluminide. Many of the more useful combinations of filament and matrix undergo reaction between the filament and the matrix if excessive temperatures are encountered during fabrication. This interaction may produce a degradation of mechanical properties. To avoid this loss of properties, the fabrication parameters employed tend to use relatively high unit pressures at the minimum practical temperatures. In the past this has effectively limited the size of the monotape to an overall surface area of less than about one square foot and often to less than about 0.5 square feet, i.e., the area of a tape which could be accommodated between the platens of a press or within the cavity of a hot isostatic pressing device. Larger sizes of monotapes may be produced by sequential pressing together of shorter lengths, but the filament alignment in uncompacted areas next to the area being pressed is frequently disturbed and may result in lower values of stiffness and strength. The time required to produce very long or very large monotapes by the sequential press bonding of shorter lengths increases directly with the required number of steps.
It has been known that to employ alloys, which react under heat and pressure, as the matrix of filament, it was necessary to seal the sandwich in an encapsulator and either evacuate the air, or inject an inert atmosphere, prior to the the step of compacting or densification.
Another problem encountered in the prior art involved attempts to subject the sandwich to a series of sequential pressing steps. It was found that the uncompacted tape had to be maintained in a substantially flat, unbent configuration because the encapsulator matrix foil and collimated filaments are relatively rigid. Consequently, if one desired to produce a ten foot long monotape, the production area had to be large enough to accommodate the uncoiled lengths on either side of the press.
Monotapes of the type contemplated by the present invention may be used to form an end product of a predetermined size and configuration, for example, very long narrow monotapes tightly coiled to form a ring or disk. The ultimate end product is formed from either a single or plurality of monotapes which are layered one over the other until the desired number of plies are present, after which the stack of monotape plies are subject to suitable forming techniques under high temperature and pressure during which the individual monotapes are in essence molded and bonded to each other and formed into the desired final configuration.
The advantage of the invention is to produce monotapes containing very long or continuous filaments for maximum strength. Monotapes of the type contemplated by the present invention may also be sectioned into a large number of smaller plies and used for a product such as a gas turbine blade. The advantage of the invention in such case is an economical method for manufacturing monotapes.
Depending upon the desired end use of the part to be formed from the monotapes, the individual monotapes may be piled one upon the other with all of the reinforcement filaments in a single parallel axis, or if desired, the filaments of one or more of the monotapes may be perpendicular to, or at an angle bias to, the axis of the reinforcing filaments in the plies above or below it.
Where the parts to be formed are wider or longer than the size of monotapes heretofore available, monotapes were generally put together very much like the layers of bricks in a building, with the joints in any given ply being offset from those in the ply above and below it. Such fabrication techniques produced a product in which the overall strength of the pieced together tapes was substantially less than could have been obtained if each monotape ran the entire length of the part. The advantage of the invention in such circumstance is to provide a monotape which may be employed without splicing or at least with a minimum of splicing.